1. Field of the Invention
The present invention relates to a crystallization apparatus, a crystallization method, a device, and a light modulation element. More particularly, the present invention relates to a technology of irradiating a non-single crystal semiconductor film with a laser beam having a predetermined light intensity distribution to generate a crystallized semiconductor film.
2. Description of the Related Art
A thin film transistor (TFT) used for, e.g., a switching element that selects a display pixel in a liquid crystal display (LCD) and others is conventionally formed by using amorphous silicon or polysilicon.
A mobility of electrons or holes of the polysilicon is higher than that of the amorphous silicon. Therefore, when the polysilicon is used to form a transistor, a switching speed is increased and response of a display thereby becomes faster as compared with a case where the amorphous silicon is used to form a transistor. Additionally, a peripheral LSI can be formed of a thin film transistor. Further, there is an advantage of reducing a design margin of any other component. Furthermore, when incorporating peripheral circuits such as a driver circuit or a DAC, these peripheral circuits can be operated at a higher speed.
Since the polysilicon is formed of an aggregate of crystal grains, when this polysilicon is used to form, e.g., a TFT transistor, a crystal grain boundary is present in a channel region of this transistor, and this crystal grain boundary serves as a barrier to reduce a mobility of electrons or holes as compared with that of single-crystal silicon. Moreover, in case of many thin film transistors formed by using the polysilicon, a number of crystal grain boundaries formed in a channel region varies depending on each of the thin film transistors, and this becomes unevenness of characteristics of the thin film transistors, resulting in a problem of display unevenness in case of a liquid crystal display. Thus, in order to improve a mobility of electrons or holes and reduce unevenness of the number of crystal grain boundaries in each channel region, a crystallization method that generates crystallized silicon having a large particle diameter enabling formation of one channel region has been recently proposed.
As this type of crystallization method, a “phase control ELA (Excimer Laser Annealing) method” of irradiating a phase shifter (a light modulation element) with an excimer laser beam and irradiating a non-single crystal semiconductor film (a polycrystal semiconductor film or a non-single crystal semiconductor film) with a Fresnel diffraction image obtained by this process or an image formed by an image forming optical system to generate a crystallized semiconductor film has been conventionally known. Particulars of the phase control ELA method is disclosed in, e.g., Surface Science, Vol. 21, No. 5, pp. 278-287, 2000.
According to the phase control ELA method, a light intensity distribution having an inverse peak pattern in which a light intensity at a point corresponding to a phase shift portion of the phase shifter is lower than that at a periphery (a pattern in which a light intensity is lowest at the center and the light intensity is precipitously increased toward the periphery) is generated, and the non-single crystal semiconductor film is irradiated with light having the light intensity distribution of this inverse peak shape. As a result, a temperature gradient is generated in a melting region in an irradiation target region in accordance with the light intensity distribution, a crystal nucleus is formed in a portion which is solidified first or a portion which is not molten in accordance with a point where the light intensity is minimum, and a crystal grows from this crystal nucleus toward the periphery in a lateral direction (which will be referred to as “lateral growth” or “grown in the lateral direction” hereinafter), thereby generating a single-crystal grain having a large particle diameter.
The present inventor has proposed a technology of forming a light intensity distribution having an inverse peak shape at a position where a light intensity is minimum in the light intensity distribution having a V-shaped pattern to radially generate crystal grains each having a very large width (see JP-A 2004-343073 (KOKAI)). Moreover, the present inventor has also proposed a technology of generating a combined distribution of a light intensity distribution having a V-shaped pattern and a light intensity distribution having an inverse peak pattern extending in one direction to generate sufficient lateral growth from a crystal nucleus along a gradient direction of the light intensity (see JP-A 2005-129915 (KOKAI)).
For example, the conventional crystallization technology disclosed in JP-A 2004-343073 (KOKAI), although crystal growth is radially carried out from an end portion of a non-melting region on a non-single crystal semiconductor film, a spread angle of the crystal growth at this moment, i.e., a radial angle defined by a pair of radially extended crystal grain boundaries is relatively narrow. As a result, a probability that the crystal grain boundary intrudes a channel region of a TFT is high, and an electric field effect mobility is lowered by carrier scattering due to this crystal grain boundary in the channel region.